Year old battery went dead after parking car for the night. recharged battery, started car ran for ten minutes, 4 hours later battery was dead. Installed new battery started car, 4 hours later dead battery. All lights were off and engine fan was off. 98 Saab 900
Something is causing an excessive current drain on the battery. Normal draw should be around 25 milliamps (.025 amps) while the car is parked. If you want to work on this yourself it best to use a meter to measure current and install it in series with the negative battery lead and check the current. You then start pulling fuses one at a time to see where the current flow goes down significantly. That fuse leg has the trouble on it. You then need to check the devices on that circuit.
Things like added accessories are a common trouble source so check them if you have any.